Certain hydroxyalkyl carbamates and polymers prepared therefrom are known in the art. For example, U.S. Pat. Nos. 4,820,830 and 5,134,205 describe hydroxyalkyl carbamate compounds prepared by reacting cyclic carbonates, for example ethylene carbonate, propylene carbonate and butylene carbonate, with selected aliphatic diamines. The hydroxyalkyl carbamates are prepared without the use of any isocyanate intermediates and can be used in coating compositions in conjunction with a crosslinking agent such as a melamine-formaldehyde resin.
Also, beta-hydroxy urethane compounds (that is, urethane compounds having a hydroxyl group in the beta position relative to the carbamoxy group) are described in U.S. Pat. No. 4,435,559. These beta-hydroxy urethane compounds are prepared by reacting an isocyanate, for example, isophorone diisocyanate and 1,6-hexamethylene diisocyanate, with a 1,2-polyol, for example, 1,2-butanediol and 1,2-hexanediol, or a combination of a 1,2-polyol and a conventional blocking agent, such as a monoalcohol. The beta-hydroxy urethane compound is prepared under such conditions that virtually no free isocyanate groups are remaining in the resultant product. These beta-hydroxy urethane compounds are useful in curable compositions as a crosslinking agent in conjunction with other composition components, or in self-crosslinkable compositions.
U.S. Pat. No. 5,340,889 describes liquid hydroxy-urethane products having cyclocarbonate end groups prepared by reacting a polyoxyalkylenediamine with a molar excess of a bis-carbonate of a bis-glycidyl ether, for example the bis-glycidyl ether of neopentyl glycol. The bis-carbonate material is reacted with the polyoxyalkylenediamine in a molar ratio ranging from 5.0:1 to 2.0:1. This reaction ratio ensures that the resulting product has cyclocarbonate end groups. The reaction is conducted at a temperature ranging from ambient to 250° C. and at a pressure ranging from atmospheric up to 3000 psig.
Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by application of a transparent or clear coat over at least a portion of the base coat have become increasingly popular as original finishes for a number of consumer products including, for example, automotive vehicles. The color-plus-clear coating systems have outstanding appearance properties such as gloss and distinctness of image, as well as excellent physical properties. Such color-plus-clear coating systems have become can be used advantageously in a variety of industrial applications including, for example automotive, aerospace, flooring and packaging applications.
Top coating systems, whether monocoats or the aforementioned color-plus-clear systems, particularly those used for automotive applications, are subject to various defects that can occur during the assembly process as well as from numerous environmental elements. On commercial automobile coating lines during application of the coating system, certain portions of the line can experience process problems. For example, the clear coat applicator might malfunction, or curing ovens can widely vary in temperature from the specification temperature. While the color coat typically is “flash cured” at a temperature sufficient to drive off solvent but not fully cure the coating, once the clear coating has been applied the color-plus-clear coating system typically is given a full bake (e.g., 250° F. (121° C.) for 30 minutes) to simultaneously cure both the base coat and the top coat. In instances where the clear coat application system is malfunctioning, the auto body with the applied color coat can continue through the clear coat applicator station and into the clear coat curing oven, thereby fully curing the color coat. If this occurs, some automobile manufacturers elect to reapply the color coat over the fully cured color coat prior to application of the clear coat. In such situations, the fully cured color coat can have poor intercoat adhesion with the subsequently applied color coat, even though the compositions may be the same.
Moreover, as discussed previously, during the assembly process, the applied color-plus-clear coating can include surface defects in the coating surface which require repair. Some automobile manufacturers elect to remove the defect and recoat the repair area with the same color-plus-clear system. In such instances, the color coating composition must be applied directly to the surface of a fully cured clear coat, followed by application of the clear coating composition over the color coating composition. It is known, however, that some clear coats when cured have poor intercoat adhesion with the subsequently applied repair color coat. This is believed to result due to the difference in surface energy of the cured clear coat and the subsequently applied repair color coating composition.
In recent years the trend in the automotive industry has been to reduce atmospheric pollution caused by the volatile solvents which are emitted during the painting process. One approach to emissions control has been the use of waterborne coating compositions as the pigmented or color coat in the color-plus-clear coating system.
U.S. Pat. Nos. 5,071,904 and 5,510,148 describe waterborne coating compositions useful for forming a base coat in color-plus-clear coating systems. The compositions comprise a polymeric film-forming resin comprising an aqueous dispersion of polymeric microparticles. The polymeric microparticles contain a substantially hydrophobic polymer which is essentially free of repeating acrylic or vinyl units in the backbone and is adapted to be chemically bound into the cured coating compositions. The remainder of the microparticle comprises an acrylic monomer or mixture of monomers. The hydrophobic polymer and the acrylic monomer(s) are particularized by high stress techniques followed by polymerization of the monomers to produce the polymeric microparticles which are stably dispersed in aqueous media.
Some waterborne coating compositions, however, are not without attendant disadvantages. For example, some waterborne coatings can have a narrow application window because it can be difficult to obtain smooth cured coatings, free of popping (described below), over a wide range of relative humidities. For example, in some instances, where the coating composition is applied in an environment having high relative humidity, water cannot readily evaporate from the applied film during the flash or dehydration period prior to curing at elevated temperatures.
Also, it has been noted that some waterborne coating compositions can increase in viscosity upon storage. Such a viscosity increase can occur for a number of reasons, for example, hydrogen bonding between various components and/or instability of rheology control additives such as associative thickeners. Such poor storage stability can necessitate the addition of water to adjust the application viscosity of the composition. This additional water decreases application resin solids of the composition, and can result in surface defects commonly referred to as “popping” which occur as water (and solvent if present) volatilizes through the coating surface upon curing.
In view of the foregoing it is desirable to provide waterborne coating compositions which have improved storage stability, excellent resistance to mottling and popping, good appearance properties, and acceptable adhesion properties.